Conventional methods for soldering, fusing or brazing articles are generally not appropriate when a large number of such operations are to be performed rapidly, as in a mass production process. Thus, for example, when mass soldering printed circuit boards having many closely spaced connections, conventional soldering iron techniques are of little use when rapid soldering must be done.
One method and apparatus which has been suggested for such purpose is described in U.S. Pat. No. 3,866,307, issued on Feb. 18, 1975 to Pfahl, Jr. et al. wherein the article to be soldered, fused or brazed is placed in hot saturated vapors generated by continuously boiling a heat transfer liquid having selected properties, including a boiling point at least equal to, and preferably above, the temperature required for a soldering, brazing or fusing operation. The vapors condense on the article and given up latent heat of vaporization to heat the article to the temperature needed for soldering, fusing or brazing. To implement such a process an apparatus for performing the desired operation can be arranged to permit a single article, a batch of articles, or a continuously moving line of articles to be subjected to hot saturated vapors in a suitable enclosure. The heat transfer liquid, for example, may be selected from a group of liquids known generically as fluorocarbons.
In a particular apparatus described in the above-referenced patent, the hot saturated vapors from the boiling heat transfer liquid of the enclosure substantially fill the enclosure. Because the fluorocarbon liquids which are used as the heat transfer liquids in such systems are relatively expensive, it is desirable to prevent the escape of any fluorocarbon vapors from the system so that substantially all of the vapors are condensed and returned to the liquid supply. Accordingly, the height of the vessel shown in the patent in which such operation takes place must be sufficiently large that the maximum level of the saturated vapors does not reach the open end of the vessel shown therein. If the enclosure is too small a substantial portion of the vapors will escape.
An improvement for reducing vapor loss in such apparatus, is disclosed in U.S. Pat. No. 3,904,102 issued to Chu et al. on Sept. 9. 1975, which patent discloses the introduction of a secondary vapor blanket in the enclosure between the primary vapors formed from the boiling heat transfer liquid of the embodiment of U.S. Pat. No. 3,866,307 and the ambient atmosphere. The primary and secondary vapors are supplied from a mixture of primary and secondary liquids in the same reservoir, the secondary vapors boiling off first at a lower temperature and filling the upper portion of the open enclosure and the more dense primary vapors which boil at a higher temperature filling the lower portion thereof. While the use of such a secondary vapor blanket tends to reduce the vapor losses therefrom, some vapors of both the primary and secondary liquids still tend to be irretrievably lost to the atmosphere from such an open system. The height of the system, while reduced somewhat from that in the Pfahl et al. patent, still has to be so large that the use of the system in many applications becomes impossible or at least highly impractical. Further, there is substantially no control of the rate of generation of the secondary vapor because the temperature of the secondary liquid cannot be controlled independent of that of the primary liquid, since a mixture of the two liquids in a common reservoir is required. As the secondary vapors from the primary/secondary liquid mixture are formed, they tend to become super heated to temperatures higher than the boiling point of the secondary liquid, the super heated secondary vapor blanket thereby tending to be less dense than it would be if such super heating did not occur. The effectiveness of the secondary vapor blanket in preventing the escape of primary vapors therethrough is accordingly not optimized. Morever, the use of an open enclosure system caused an unreasonable loss of both the primary and the secondary vapors and a consequent reduction in the overall effectiveness of the system.
Even if a lid is placed on the top of the enclosure in such a system, escape of both vapors from the system will occur when the lid is removed to permit insertion of the article to be treated, particularly where the area of the opening is relatively large.
In order to overcome the disadvantages of the above systems, it is desirable that a hot saturation vapor apparatus for soldering, fusing or brazing articles be designed so that its overall height can be minimized without reducing the efficiency of the device, i.e., while still substantially preventing the escape of any expensive fluorocarbon capors therefrom. It is further desirable that initial manufacturing and installation costs, as well as the continuing operating and maintenance costs of the apparatus be kept to reasonable levels.